The proposed research is subdivided into two areas, both of which utilize the goldfish Mauthner cell (M-cell) as a model. In the first aspect, basic mechanisms of synaptic interactions will be investigated. Specifically, the physiological and morphological properties of identified interneurons mediating both field effect and chemical inhibitions of the M-cell will be studied, as will the role of the axon cap and its glial cell border in the generation of the field effects and the local regulation of membrane excitability. In addition, noise analysis will be used to determine the kinetic properties of single inhibitory conductance channels. The input-output relations of the mixed electrotonic and chemical synapses between the M-cell lateral dendrite and VIIIth nerve afferents will be studied with simultaneous intracellular recordings, and the effect of intracellular Ca ions ion injection on these mixed synapses will be determined. Finally, the hypothesis that the M-cell has Ca ions-dependent dendritic spikes which could modulate the properties of these synapses will be tested. These experiments will also be designed to study the functional organization of the local medullary networks involving the M-cell. The second aspect deals will the response of the M-cell to partial deafferentation. Intracellular recordings will be used to determine alterations in M-cell membrane properties and synaptic interactions following transection of different VIIIth nerve branches. The results will be correlated with light and electron microscopic morphological studies. Selective deafferentation will permit differentiation of the trophic functions of electrotonic and chemical synapses. The role of intra- and interneuronal transport functions, particularly dendritic transport, in the maintenance of membrane properties and synaptic connectivity will also be investigated, using the techniques of intracellular injections of labeled metabolic substances both in control situations and after deafferentation.